1. Field of the Invention
The subject of the present invention is novel packing structures and their production process that are characterized in that the drying step is controlled by its operating parameters, namely the drying thermal cycle (temperature rise and drop rates, temperatures, times).
2. Related Art
It is known to use containers under pressure, containing gases such as acetylene, dissolved in a solvent such as acetone, in various medical and artisanal applications and especially to carry out welding, brazing and heating operations together with an oxygen bottle.
These containers are usually packed with solid filling materials intended to stabilize the gases that they contain, which are thermodynamically unstable under the effect of variations in pressure or temperature, and therefore liable to decompose during their storage, transport and/or distribution.
These materials must be sufficiently porous so as to make it easy to fill and release the gases contained in the container. They must also be incombustible and inert with respect to these gases and have good mechanical strength. These materials conventionally consist of porous silico-calcareous ceramic substances obtained for example from a homogeneous mixture, in water of quicklime or milk of lime and silica (especially in the form of quartz flour), as described in the documents WO-A-93/16011, WO-A-98/29682 and EP-A-262031, so as to form a slurry, which then undergoes a hydrothermal synthesis. Specifically, the slurry is introduced into the container to be packed, under a partial vacuum, which is then autoclaved at a certain pressure and temperature, and then dried in an oven so as to completely remove the water and form a monolithic solid mass of composition CaxSiyOz,wH2O_, having crystalline structures of the tobermorite and xonotlite type, possibly with residual quartz present. Various additives may be introduced into these mixtures of the prior art in order to improve the dispersion of the lime and silica and thus avoid forming structural inhomogeneities and shrinkage phenomena observed during the hardening of the porous mass. The filler materials obtained must in fact have a homogeneous porosity with no empty spaces, within the material and between the material and the container, in which empty spaces gas pockets could accumulate and run the risk of causing an explosion.
Document EP-A-264550 also indicates that a porous mass containing at least 50%, or at least 65% or even at least 75% by weight of crystalline phase (with respect to the weight of calcium silicate) makes it possible to meet the two requirements of compressive strength and resistance to shrinkage at the hydrothermal synthesis and firing temperatures.
Although the known porous masses are generally satisfactory from the standpoint of their mechanical strength, the fact remains that the properties of withdrawing gases trapped in these porous masses are at the present time insufficient and/or completely random. This random aspect is due to the lack of control of the phases formed and of the microstructure of the porous mass, due to the lack of control/understanding of the process and especially the hydrothermal synthesis step by controlling the operating parameters, namely the temperature rise rate, the synthesis temperature, the duration of the temperature hold and control of the cooling rate.
Indeed, depending on the operating conditions of the bottles (use temperature, work rate, amount of gas contained in the bottle, etc.), they do not always allow the gas that they contain to be continuously withdrawn, at a high flow rate, throughout the duration needed for certain applications, especially welding applications, with a maximum gas recovery rate, corresponding to the ratio of the amount of gas that can be used to the amount of gas initially stored. Now, it would be desirable to be able to satisfy a flow rate of 200 l/h continuously for 15 minutes and a peak flow rate of 400 l/h for 4 minutes, for a gas capacity equal to or greater than 50% at the start of the test (defined as the ratio of the amount of gas present at this instant to the amount of gas initially loaded into the container), the container having a diameter/length ratio of between 0.2 and 0.7, preferably between 0.35 and 0.5, for a minimum water capacity of one liter and preferably between 3 and 50 liters.
This insufficiency is due in particular to the thermal loss associated with extracting the gas from the solvent, which may prove to be very prejudicial to gas withdrawal. This thermal loss is not due mainly to the intrinsic conductivity of the silico-calcareous material (as a reminder, the void content is between 87 and 92%) but to the size (dimensions) of the needle-shaped crystals constituting the porous mass. This is because the smaller their size, (i) the larger the number of points of contact between them and ii) the lower the d50 of the pore size distribution (d50 is defined as the average spread of the pore distribution). This therefore handicaps conductive heat transfer, leading to a relatively long period of “unavailability of the bottle”. This effect is to be correlated with the pore distribution. In the case of an acetylene bottle for example, the energy consumption is of the order of 600 joules per gram of acetylene extracted from the solvent. In practice, this results in the bottle being cooled considerably during withdrawal, leading to greater solubilization of the acetylene in the solvent and thus a drop in pressure, with repercussions on the withdrawal rate. The flow is finally exhausted when the pressure at the bottle outlet falls below atmospheric pressure.